Effect of evaporation and agglomeration on droplet shape oscillations

• Wavelet transform reveals non-linear signatures in water droplet oscillation.
• Continuous increase in natural frequency of evaporating ethanol droplet.
• Drastic reduction in oscillation level due to particle aggregation.

The effects of evaporation and the presence of agglomerating nanoparticles on the oscillation characteristics of pendant droplets are studied experimentally using ethanol and aqueous nanoalumina suspension, respectively. Axisymmetric oscillations induced by a round air jet are considered. Wavelet transform of the time evolution of the 2nd modal coefficient revealed that while a continuous increase in the natural frequency of the droplet occurs with time due to the diameter regression induced by vaporization in the case of ethanol droplet, no such change in resonant frequency occurs in the case of the agglomerating droplet. However, a gradual reduction in the oscillation amplitude ensues as the agglomeration becomes dominant.